Volt Drop

elementps · Jun 1, 2013

Hi all,

Just wanting to clarify regarding volt drop.
Client is looking to add lighting to garden, to be supplied from 13a FCU, load on FCU will be less than 1A due to LED lighting.
The FCU will be an addition to a final ring circuit which serves dining room and lounge, protected by 32A MCB, actual load is probably no more than 13A.

Im getting myself confused with volt drop requirements, the volt drop on the final ring is within requirements for its load.

Now the volt drop for the garden lighting, is this taken as just the impedance from the FCU (13A fuse) or is it impedance of the ring plus the leg from FCU.

Let me know if i need to be more clear or specific?

Thanks in advance.

My back up plan is to feed the lighting from a dedicated circuit from a DB located at other end of the garden.

telectrix · Jun 1, 2013

work out your VD on your installation from the FCU. then add to the VD of the RFC. that will give you the total VD for your modified circuit.

Richard Burns · Jun 1, 2013

Volt drop is calculated from the origin to the furthest point of the circuit. So for you lighting circuit you will need two values the volt drop of the ring and the volt drop of the lighting spur.

For the ring you have to assume a "standard" value of 26A (osg 7.2.2) since you do not know the items that may be connected.
length of ring over 4 (double conductors in both directions) and design current of 26A allows you to calculate the volt drop with a volt drop of 0.018 V/m/A for 2.5 mm conductors.

add the colt drop for the spur and you can determine if this meets the minimum 6.9V required.

elementps · Jun 3, 2013

Thanks for clarification guys.

So if I was to run from a sub main is the total vd back to origin of supply or back to protective device at local sub main db?

Sintra · Jun 3, 2013

All volt drop calcs have to be taken from the origin. If you are installing from a sub board then the volt drop of the sub main has to be included for any final circuits from the sub board.

Landril · Jun 5, 2013

Why are you worrying about VD from the house to the back yard - how long is the run ..size of a football field!
Also you may find that LED drivers normaly have a voltage rating from 110v - 240v A.C and as they normal are run from a constant current a slight voltage drop is not normaly an issue.

Also is the current FRC protected by a RCD as you will need the outside lighting to be!